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VACUUM()			 SQL Commands			      VACUUM()



NAME
       VACUUM - garbage-collect and optionally analyze a database


SYNOPSIS
       VACUUM [ FULL | FREEZE ] [ VERBOSE ] [ table ]
       VACUUM [ FULL | FREEZE ] [ VERBOSE ] ANALYZE [ table [ (column [, ...] ) ] ]


DESCRIPTION
       VACUUM  reclaims	 storage  occupied by deleted tuples.  In normal Post-
       greSQL operation, tuples that are deleted or obsoleted by an update are
       not  physically	removed	 from their table; they remain present until a
       VACUUM is done. Therefore it's necessary	 to  do	 VACUUM	 periodically,
       especially on frequently-updated tables.

       With  no	 parameter,  VACUUM  processes	every  table  in  the  current
       database. With a parameter, VACUUM processes only that table.

       VACUUM ANALYZE performs a VACUUM and then an ANALYZE for each  selected
       table.  This  is	 a  handy  combination	form  for  routine maintenance
       scripts. See ANALYZE [analyze(7)] for more details about	 its  process-
       ing.

       Plain  VACUUM  (without FULL) simply reclaims space and makes it avail-
       able for re-use. This form of the command can operate in parallel  with
       normal  reading	and  writing of the table, as an exclusive lock is not
       obtained. VACUUM FULL does more extensive processing, including	moving
       of tuples across blocks to try to compact the table to the minimum num-
       ber of disk blocks. This form is much slower and requires an  exclusive
       lock on each table while it is being processed.

       FREEZE  is  a  special-purpose  option  that causes tuples to be marked
       ''frozen'' as soon as possible, rather  than  waiting  until  they  are
       quite old. If this is done when there are no other open transactions in
       the same database, then	it  is	guaranteed  that  all  tuples  in  the
       database	 are  ''frozen''  and  will  not  be subject to transaction ID
       wraparound problems, no matter how long the database  is	 left  unvacu-
       umed.   FREEZE  is  not	recommended for routine use. Its only intended
       usage is	 in  connection	 with  preparation  of	user-defined  template
       databases,  or  other  databases that are completely read-only and will
       not receive routine maintenance VACUUM operations.  See the  documenta-
       tion for details.

PARAMETERS
       FULL   Selects ''full'' vacuum, which may reclaim more space, but takes
	      much longer and exclusively locks the table.

       FREEZE Selects aggressive ''freezing'' of tuples.

       VERBOSE
	      Prints a detailed vacuum activity report for each table.

       ANALYZE
	      Updates statistics used by the planner  to  determine  the  most
	      efficient way to execute a query.

       table  The  name	 (optionally  schema-qualified) of a specific table to
	      vacuum. Defaults to all tables in the current database.

       column The name of a  specific  column  to  analyze.  Defaults  to  all
	      columns.

OUTPUTS
       When  VERBOSE  is specified, VACUUM emits progress messages to indicate
       which table is currently being processed. Various statistics about  the
       tables are printed as well.

NOTES
       We  recommend  that  active production databases be vacuumed frequently
       (at least nightly), in order to remove expired rows.  After  adding  or
       deleting	 a large number of rows, it may be a good idea to issue a VAC-
       UUM ANALYZE command for the affected table. This will update the system
       catalogs	 with  the  results of all recent changes, and allow the Post-
       greSQL query planner to make better choices in planning queries.

       The FULL option is not recommended for routine use, but may  be	useful
       in  special cases. An example is when you have deleted most of the rows
       in a table and would like the table to physically shrink to occupy less
       disk space. VACUUM FULL will usually shrink the table more than a plain
       VACUUM would.

       VACUUM causes a substantial increase in I/O traffic,  which  can	 cause
       poor  performance for other active sessions. Therefore, it is sometimes
       advisable to use the cost-based vacuum delay feature.  See the documen-
       tation for details.

EXAMPLES
       The  following  is  an  example	from  running VACUUM on a table in the
       regression database:

       regression=# VACUUM VERBOSE ANALYZE onek;
       INFO:  vacuuming "public.onek"
       INFO:  index "onek_unique1" now contains 1000 tuples in 14 pages
       DETAIL:	3000 index tuples were removed.
       0 index pages have been deleted, 0 are currently reusable.
       CPU 0.01s/0.08u sec elapsed 0.18 sec.
       INFO:  index "onek_unique2" now contains 1000 tuples in 16 pages
       DETAIL:	3000 index tuples were removed.
       0 index pages have been deleted, 0 are currently reusable.
       CPU 0.00s/0.07u sec elapsed 0.23 sec.
       INFO:  index "onek_hundred" now contains 1000 tuples in 13 pages
       DETAIL:	3000 index tuples were removed.
       0 index pages have been deleted, 0 are currently reusable.
       CPU 0.01s/0.08u sec elapsed 0.17 sec.
       INFO:  index "onek_stringu1" now contains 1000 tuples in 48 pages
       DETAIL:	3000 index tuples were removed.
       0 index pages have been deleted, 0 are currently reusable.
       CPU 0.01s/0.09u sec elapsed 0.59 sec.
       INFO:  "onek": removed 3000 tuples in 108 pages
       DETAIL:	CPU 0.01s/0.06u sec elapsed 0.07 sec.
       INFO:  "onek": found 3000 removable, 1000 nonremovable tuples in 143 pages
       DETAIL:	0 dead tuples cannot be removed yet.
       There were 0 unused item pointers.
       0 pages are entirely empty.
       CPU 0.07s/0.39u sec elapsed 1.56 sec.
       INFO:  analyzing "public.onek"
       INFO:  "onek": 36 pages, 1000 rows sampled, 1000 estimated total rows
       VACUUM


COMPATIBILITY
       There is no VACUUM statement in the SQL standard.

SEE ALSO
       vacuumdb [vacuumdb(1)], the documentation



SQL - Language Statements	  2010-12-14			      VACUUM()
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